PrefixSuffixSaver is an io.Writer which retains the first N bytes
and the last N bytes written to it. The Bytes method reconstructs
it with a pretty error message.
It is copied from os/exec/exec.go of the Go stdlib.

Bytes returns a slice of the bytes, or a copy of the bytes, retained by w.
If more bytes than could be retained were written to w, it returns a
concatenation of the N first bytes, a message for how many bytes were dropped,
and the N last bytes.